1. Field
The present disclosure relates generally to data networking and in particular to radar detection with a radio transceiver for connecting remote edge access networks to core networks.
2. Related Art
Connections to remote edge access networks from core networks are often achieved with wireless radio, wireless infrared, and/or copper wireline technologies. Radio, especially in the form of cellular or wireless local area network (WLAN) technologies, is particularly advantageous for supporting mobility of data networking devices. However, cellular base stations or WLAN access points inevitably become very high data bandwidth demand points that require continuous connectivity to an optical fiber core network. When data aggregation points, such as cellular base station sites, WLAN access points, or other local area network (LAN) gateways, cannot be directly connected to a core optical fiber network, then an alternative connection, using, for example, wireless radio or copper wireline technologies, must be used. Such connections are commonly referred to as “backhaul.” The array of network backhaul and other high throughput radio applications include point-to-point, point-to-multipoint, networks of multiple point-to-point and multipoint links, ad hoc, ring, self-organizing and mesh networks. These network architectures, often using directive antennas, are needed to support wireless last mile hops and wireless backhaul applications that are used to bring high throughput services to cellular telephone systems and broadband services to enterprises and the home.
Increasingly, high throughput services to consumers and business are becoming one of the cornerstones of future economic vitality. It is very efficient to support these high-demand needs using frame-based transmission links, and particularly so with very high duty factor transmission frequency domain duplexing (FDD). A frame-based system refers to radios with continuous or near-continuous time transmission where time is divided into frames. Each frame carries channel estimation and control information, as well as multiple opportunities to stuff incoming packet-based data onto the frame. When there is no network data available, the frame is transmitted with dummy data blocks. This allows the dropping of incoming data into the data blocks with very low latency and high reliability. The low latency comes from the fact that the link is already running and just needs to substitute the incoming data for the dummy data. The high reliability comes from the fact that the link can be set up and maintained over a period of time that is longer than what would be efficient in a packet radio link. This view of frame-based operation is consistent with the definition of “frame based” in the European ETSI standard EN 301 893.
Radio signals can interfere with radar reception. Because the protection of radar operation is important, regulatory bodies control channel access of radio systems that share the band with radars and set up radar signal level limits for detection of various types of radar signals. In many regulatory regions, a transmitted radar signal that is detected by a radio's radar detector at −64 dBm is considered too close to the radio system and the radio system must cease transmitting on that channel and move to another channel. A channel is the occupied bandwidth of the data transmission stream over the RF link. But a channel may comprise more than one noncontiguous part where each part can have a different center frequency and occupied bandwidth. The channel frequency of each part is its center frequency.
Although effective for high performance communications, FDD frame-based radio operation makes detection of radar signals using conventional methods impossible. In effect, frame-based transmission is similar to circuit switched operation of the older telephone circuit switching technology. In an FDD link, frame based transmitters can operate at up to 100% transmit duty factor because the responses for each channel arrive in the other channel—a single transceiver can operate with up to 100% transmit duty factor in one channel and up to 100% receive duty factor in another channel, while the transceiver on the other end of the communication link does the opposite on the channels. Various implementations may use less than 100% duty factor to, for example, sense the channel to satisfy a channel sharing regulation or system wide self-interference requirements. But, in each case, the duty factor is significantly higher than a packet radio system and can approach 100% in many cases. Thus, with typical FDD radios, the transmitters on both ends of an FDD link usually transmit together coincident in time for at least some fraction of every frame. When the transmit duty factor is high, this means that radar detection for one or both of the transmit directions is preferably performed within a fraction of every frame that includes this fraction where both ends of an FDD link transmit together coincident in time.
In bands that require radar detection, there is typically a sequence of stages that a transmitter goes through before and at the beginning of operation. In the first stage, prior to operation, the transmitter determines whether the channel is clear of radar transmissions. The regulatory agencies call this stage Channel Availability Check (CAC). In CAC, the radar detector informs the transmitter that a channel cannot be used if certain types of radar are detected. If such radars are detected, the channel is typically off limits for 30 minutes, at which time another CAC must be performed. The next stage is link operation. In the link operation stage, the receiver detects and acquires the transmission and a round trip connection is made (for full duplex). Often, the receiver and transmitter negotiate an operating frequency. During this period of time, which may be part of a radio channel “bootstrap” sequence, radar monitoring may continue. Typically, there is a 200 ms regulatory requirement for a transmitter to stop transmitting after a radar pulse sequence occurs on a channel. If the bootstrap sequence is a significant duration on the scale of 200 ms, then, under most regulatory requirements, radar monitoring is still needed. After the bootstrap sequence, normal run-mode operation can occur. During this time, in-service monitoring for the radar occurs. The in-service monitoring requires the detection of the presence of certain types of radar on an operating channel and closing the transmission within 200 ms (i.e. the required close time) of the end of the radar pulse train that is used to certify this operation in testing. It should be noted that over the course of a radio's operating time, it may re-enter the bootstrap mode and the normal run-mode operation multiple times, particularly if the synchronization between the radios is disrupted, or even as a normal maintenance operation.
One problem with existing networks is that when the transmitter is transmitting, the high signal level swamps out the receive signals for typical receivers that are located in close proximity to the transmitter, thereby limiting the ability for a radar detector co-located with the transmitter to detect a radar signal.
Packet radio systems, such as WiFi, handle this by testing their radar detection operation while transmitting at low duty factor, typically much less than 40% transmission period. These packet radio systems detect the radar with a co-located detector at the transmitter while the transmitter is not sending and the radio is available to receive or is receiving. The channel monitoring applies to the next time the transmitter operates. If a WiFi system operates with a transmit traffic load such that it uses a high duty factor, it can miss radar detections.
US20070264935 to Behzad Mohebbi, assigned to Nextivity, describes a bi-direction FDD link for use in the 5 GHz USA UNII-2 band, which requires radar detection. Because the Mohebbi disclosure is bi-directional in nature, it first transmits FDD in one direction on frequency channel 1, while receiving on frequency channel 2, then switches so that the same radio that was transmitting on channel 1 now transmits FDD on channel 2 and receives on frequency channel 1. Although the radar detector for the local transmitter is co-located with the transmitter, the detection is performed on the transmitter channel during the half cycle period that the transceiver is receiving on that channel for the forthcoming transmission. In this way, the Mohebbi disclosure is closely related to WiFi, which performs radar detection when in receive mode for forthcoming transmissions; Mohebbi differs in that it is not performing radar detection for the transmitter that is operating on the other end of the link. Mohebbi is more accurately described as a pair of TDD links, each on one frequency, which have anti-phase transmit/receive cycles. In other words, there is a TDD transmission between radios on channel 1 and another on channel 2, but at each transceiver, it transmits on channel 1 while receiving on channel 2 and vice versa. In Mohebbi, each transceiver transmits a first portion to the other on a frequency 1, while receiving from the other on frequency 2, and radar detecting on a frequency 2; and, each transceiver transmits a second portion to the other on the frequency 2, while receiving from the other on the frequency 1, and radar detecting on frequency 1. Therefore, each transceiver in Mohebbi performs radar detection on both frequency 1 and frequency 2 during the receive period for that frequency to enable the transmission on that frequency on the opposite TDD cycle, similar to the way a WiFi packet radio radar detector operates. It cannot perform radar detection on the transmit channel while the transmitter is transmitting. The Mohebbi system is also inferior in radio performance because it requires a clear channel available in both directions for two frequencies, which is very difficult to achieve from a frequency planning point of view. Additionally, the UNII-2 band is a commonly used unlicensed band that has no controls on interference. The best performance occurs in channels that show the lowest interference levels; interference is a receiver phenomenon. The level of interference measured at the transmitter is uncorrelated to that measured at the receiver simply because there is different propagation from an arbitrarily placed interference source to each side. It is only the interference level at the receiver and not the transmitter that matters because that is what causes reduced signal to interference levels. If a transceiver must receive at two frequencies to maintain a link, it must find two channels that are relatively free of interference to operate, making it much harder to create a good link; in a probabilistic interference setting, this requirement, at a minimum, squares the difficulty of operation. Moreover, in the system of Mohebbi these two channels must be clear at both transceivers on either end of the link and for backhaul radios separated by considerable distances this also squares the difficulty of operation because the interference environments at each end of the link are likely quite different. Also, since the receiving period is the time when the radar detection for the transmitter must be performed in Mohebbi, the transmit and receive channels for one of the pair of “first and second portions” must be the same. That is why it is effectively a pair of TDD channels.
In a WiFi application, the transmissions are packetized and the transmitters generally operate at a low duty factor. In fact, when the transmitters are tested for regulatory compliance for radar detection, they operate at less than a 40% duty factor. Packet radio systems are able to detect radars with the radar detector co-located with the transmitter because they detect the radar signals during the typically greater-than-60% of the time the transmitter is not transmitting.
Thus, the frame-based FDD system has particular challenges for performing radar detection under the various regulatory requirements around the world because, unlike packet-based transmitters such as WiFi radios that can operate at a modest duty factor, the frame-based transmitter is active at high duty factors. There is no opportunity for performing in-service monitoring local to the transmitter under the conventional art because the detection mechanism must listen for signals at −64 dBm while the transmitter is operating on the same channel at, for example, +30 dBm or higher and at nearly 100% duty factor.
By most regulations, a channel which requires radar detection cannot be occupied before completing a 60 second listen period for radars (CAC). If a radio transceiver is forced to vacate its operating channel and it does not have another channel queued up that it has already performed a successful CAC on, it will have to remain off the air for at least 60 seconds. An outage of this length is unacceptable in many applications.
Also, it is often a regulatory requirement that when a radar is detected in a channel, at least 80% of the occupied bandwidth of the channel must be vacated and must remain unused by the detecting system for at least 30 minutes, despite the actual receiver operating bandwidth of the radar. But for wideband devices, as are often found in high-duty-factor links that achieve high throughput, this unduly punishes a system for spreading its channel power over a wider bandwidth, thus reducing its spectral power density.